1. Field of the Invention
The present invention relates to a passenger seat device that includes a detector for detecting whether a seat is occupied, and to a method of manufacturing the same.
2. Description of Related Art
JP-A-2003-80989 proposes a device for detecting whether a passenger seat of an automotive vehicle is occupied or not. In this device, a passenger on the seat is detected based on changes in an alternating current electromagnetic field in the vicinity of the seat, particularly at a portion above the seat. This type of the detector is referred to as a capacitance-type detector. The detector includes a transmitting electrode embedded in the seat for forming the electromagnetic field and a receiving electrode embedded in a seat-back. A capacitive impedance between the transmitting electrode and receiving electrode decreases when a passenger occupies the seat. Thus, whether the seat is occupied or not is detected.
The capacitance-type detector described in JP-A-2003-80989 also includes a seat heater embedded in the seat at a position under the transmitting electrode (also referred to as a detecting electrode). When both of the seat heater and the occupant detector are embedded in a seat as in this example, the seat heater having a substantially constant potential under alternating current is positioned in the vicinity and under the detecting electrode.
In this detector, there is a problem that a large capacitance (referred to as a seat heater capacitance) is formed between the detector electrode and the seat heater because both are positioned close to each other, facing each other in a large area. An alternating current impedance for detecting a passenger is a compound impedance (mostly capacitance) composed of an impedance between the detecting electrode and a passenger and another impedance between the passenger and a vehicle body (a ground potential), both impedances being connected in series. Accordingly, the compound impedance becomes considerably large compared with an impedance due to the seat heater capacitance.
Since the compound impedance and the impedance due to the seat heater capacitance are connected in parallel to the detector electrode, detection sensitivity of the detector is considerably decreased due to presence of the heater impedance. In addition, the seat heater hinders flow of electric flux lines between a lower surface of the detecting electrode and a passenger due to a shielding effect of the seat heater. JP-A-2003-80989 proposes, to cope with these problems, to dispose a flat sheet electrode having a floating potential between the detecting electrode and the seat heater. However, it has been found out that the detecting sensitivity is hardly improved because the seat heater capacitance is little decreased by the presence of the sheet electrode having a floating potential.